Margin of error: NVE evaluation of February avalanche says better data and more caution needed when evaluating risk

Get used to serious storms more frequently and evacuations after them as officials rely a bigger safety margin in determining if there is a risk of avalanches or landslides hitting structures.

A report released Thursday assessing the circumstances of an avalanche Feb. 21 that destroyed two apartment buildings near the center of Longyearbyen urges better collection of snow and weather data, erring on the side of caution when evaluating it, and realizing climate change means past historical trends may no longer be relevant.

“Previous snow and avalanche history does not necessarily give the full picture of today’s situation,” the report by The Norwegian Water Resources and Energy Directorate (NVE) states. “In a short period of time there have been two avalanches that hit buildings where avalanches haven’t reached before. This indicates that the conditions are challenging and that there must be a good margin of safety in the future.”

A decision not to evacuate the Lia neighborhood below Sukkertoppen –as well as others in at-risk areas near mountainsides – was made hours before the avalanche when NVE experts declared any snowslide wouldn’t be large enough to hit structures. The resulting avalanche shook the faith of residents and local leaders in an avalanche warning system established after an avalanche in the same area on Dec. 19, 2015, that destroyed 11 homes and killed two people.

The governor ordered the indefinite evacuation of more than 60 residences following the February avalanche and officials are now determining when and if they might be reoccupied if snow barriers or other protection are built – although many residents have said they no longer want to live there.

The NVE report states the recommendation not to evacuate was based on faulty assumptions. It notes the avalanche was triggered from the top of the mountain and, while part of the snow was trapped by an outcrop, the remaining snow triggered a new slide further down.

“No avalanches have been observed previously from the top of the mountain, but the scenario was foreseen in previous reports about the avalanche situation in Lia,” the report states.

The assumption an avalanche would not be triggered from the top of the mountain was a key factor in the pre-slide recommendation not to evacuate residences.

“It was assumed that the snow on the peak would be blown away in the current weather situation,” the report states. “The significance of the edge grain layer may have been underestimated.”

“The big picture, the region forecast for (the vicinity of Longyearbyen), began with an evacuation of the most vulnerable buildings as a starting point, but after a local assessment it was not expected that avalanches would be large enough to reach settlement,” the report states.

Among the report’s primary recommendations:

• Local alerts should place greater emphasis on their uncertainty. “Focusing on details can pose a risk of believing in greater safety in the assessments than there actually are. Insufficient information should always be considered as a sign of uncertainty and included in the security margin of the notification.”

• Local alerts should have a better quality control system. “More information on weather and snow cover can provide a better database for assessing local conditions and therefore less uncertainty.”

• The alerts should give greater consideration to possibility the danger may increase rapidly with the combination of strong wind and loose snow, and climate change means the storms may be more severe than in past years.